1. Field of the Invention
The present invention relates to an ion exchange membrane for use as a diaphragm for cells and dialysis and for use in sensors, specifically an ion exchange membrane advantageously used as a diaphragm for a direct methanol type fuel cell and to a production process therefor.
2. Description of the Prior Art
An ion exchange membrane is widely used as a diaphragm for cells such as solid polymer type fuel cells, redox flow cells and zinc-bromine cells and as a diaphragm for dialysis. Out of these, a solid polymer type fuel cell which comprises an ion exchange membrane as an electrolyte is one of clean and highly efficient generation systems which take out chemical energy generated from a reaction between continuously supplied fuel and an oxidizing agent as electric power and its importance is growing in the fields of automobiles, home electric appliances and portable devices because it operates at a low temperature and is small in size. The solid polymer type fuel cell is generally constituted such that a gas diffusion electrode carrying a catalyst is joined to both sides of a solid polymer diaphragm which functions as an electrolyte. Fuel which is a hydrogen gas or methanol is supplied into a chamber (fuel chamber) where one of the gas diffusion electrodes is existent whereas oxygen or an oxygen-containing gas such as air is supplied as an oxidizing agent into a chamber where the other gas diffusion electrode is existent, and an external load circuit is connected between the gas diffusion electrodes. Thus the solid polymer type fuel cell serves as a fuel cell. A direct methanol type fuel cell which uses methanol as a direct fuel is easy to handle as the fuel is liquid, inexpensive and expected to be used as a relatively low output power source for portable devices.
The basic structure of this direct methanol type fuel cell is shown in FIG. 1. In FIG. 1, (1) denotes a cell interstructure, (2) a fuel flow hole, (3) an oxidizing gas flow hole, (4) a diffusion electrode on the fuel chamber side, (5) a gas diffusion electrode on the oxidizing agent chamber side, and (6) a solid polymer electrolyte membrane. In this direct methanol type fuel cell, protons (hydrogen ions) and electrons are produced from methanol supplied into a fuel chamber (7) on the fuel chamber side diffusion electrode (4), and the protons move to the oxidizing agent chamber (8) through the solid polymer electrolyte (6) to be reacted with oxygen contained in air or oxygen gas to produce water. At this point, electrons produced on the fuel chamber side diffusion electrode (4) move to the oxidizing agent chamber side gas diffusion electrode (5) through the external load circuit, thereby obtaining electric energy.
In the direct methanol type fuel cell having the above structure, a cation exchange membrane is generally used as the above diaphragm. The cation exchange membrane must have low electric resistance, high physical strength and low permeability of methanol used as fuel. When an ion exchange membrane having high methanol permeability is used as a diaphragm for a fuel cell, it is impossible to completely prevent methanol in the fuel chamber from being diffused into the oxidizing agent chamber and it is difficult to obtain high output from the cell.
A perfluorocarbon sulfonic acid membrane has been mainly used as the cation exchange membrane used as a diaphragm for a direct methanol type fuel cell. Although this membrane has excellent chemical stability, it does not have sufficiently high physical strength, thereby making it difficult to reduce electric resistance by decreasing the membrane thickness. In addition, when methanol is used as fuel, the perfluorocarbon sulfonic acid film greatly expands and deforms and the diffusion of methanol into the oxidizing agent chamber cannot be completely suppressed. Further, the perfluorocarbon sulfonic acid film is very expensive.
Further, it is proposed that a cation exchange membrane having low electric resistance and extremely low gas permeability may be manufactured as a diaphragm for a solid polymer type fuel cell by impregnating and polymerizing a polyolefin-based or polyfluorocarbon-based resin porous film with a monomer having a functional group capable of introducing a cation exchange group (see JP-A 2001-135328 and JP-A 11-310649) (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). However, when the cation exchange membrane is used as a diaphragm for a solid polymer type fuel cell which uses a hydrogen gas as fuel, the above effect is observed whereas when it is used as a diaphragm for a direct methanol type fuel cell, methanol permeability cannot be completely suppressed, thereby causing the diffusion of methanol into the oxidizing agent chamber with the result of deteriorated cell performance.
Meanwhile, in order to improve the water retention properties and ion conductivity of the ion exchange membrane, there is proposed a film prepared by uniformly dispersing silica particles or silica whiskers into an ion exchange membrane (for example, JP-A 6-111827). Although water retention properties and ion conductivity are improved by this method, studies conducted by the inventors of the present invention reveal that methanol permeability does not change or becomes worse in a case.
Thus, a cation exchange membrane which has low methanol permeability and low electric resistance (high ion conductivity) has been unknown as the cation exchange membrane which is used as a diaphragm for a direct methanol type fuel cell.